Recited in Patent Literature 1 is one example of related method and system for detecting a position of an electronic pen by using ultrasonic. The position detection system, which comprises an electronic pen having a function of transmitting an ultrasonic signal and an infrared trigger signal of a fixed waveform in a fixed cycle, a receiver for receiving a transmitted signal and software, measures a propagation time of ultrasonic from the electronic pen corresponding to a received trigger signal and determines a position of the electronic pen by using the propagation time. Ultrasonic transmitted from the electronic pen, which is a signal whose waveform is as shown in FIG. 15, is transmitted with the same waveform in a fixed transmission cycle. When receiving an infrared trigger signal, a reception side receives an ultrasonic signal arriving with a delay according to a propagation distance.
Patent Literature 1: U.S. Pat. No. 6,118,205.
While an infrared trigger signal transmitted from an electronic pen has only a direct wave because it arrives at a receiver through a single path, an ultrasonic signal will have a reflected wave because it might reflect on a surrounding wall or the like to arrive at a receiver through a plurality of paths whose propagation distance is different. Therefore, as shown in FIG. 15, the ultrasonic will have not only a direct wave 1 of ultrasonic to be originally detected but also a reflected wave 1. Since the reflected wave 1 has a propagation distance longer than that of the direct wave 1, it arrives at a receiver at t3 immediately before an arrival time t4 of a subsequent direct wave 2. Accordingly, the receiver will detect the ultrasonic signal not at the arrival time t4 but the arrival time point t3 of the reflected wave 1, so that it is impossible to calculate an accurate position of the electronic pen. The first problem of the above-described related art is therefore that when the receiver receives, after receiving an infrared trigger signal, not a direct wave of an ultrasonic signal transmitted simultaneously with the trigger signal but a reflected wave which propagates on a path larger than a path length of a direct wave transmitted in a preceding cycle, it erroneously recognizes the received reflected wave as a direct wave, so that measurement of a propagation time will have an error.
The second problem is that because discrimination between a direct wave and a reflected wave is extremely difficult, when the receiver receives a composite wave with a direct wave and a reflected wave of an ultrasonic signal after receiving a trigger signal, it is difficult to precisely extract a direct wave from the received composite wave.
An object of the present invention is to provide electronic pen position detecting method and system using ultrasonic which eliminate effects of a reflected wave of an ultrasonic signal sent from an electronic pen and enable accurate measurement of a propagation time of a direct wave that arrives fastest in each cycle of the ultrasonic signal sent from an ultrasonic emission source of the electronic pen without being affected by the reflected wave of the ultrasonic signal.